1. Field of the Invention
The present invention relates to a digital intermediate frequency transmitter for a wireless communication, and in particular, to a digital intermediate frequency transmitter for a wireless communication wherein an improvement of a modulation quality such as an EVM, a linearity and a power consumption is possible, and a main characteristic of a wireless communication transmitter may be embodied at a low cost through an integration such that a high frequency 90° phase shifter, a voltage controlled oscillator having a 2× frequency or higher and a high frequency I/Q local oscillated signals are not required and a side-band signal may be effectively removed.
2. Description of Prior Art
Recently, a research on a development of a one-chip solution for reducing a power consumption of a wireless communication system is in progress.
A transmitter used in a conventional wireless communication system employs a superheterodyne method. In accordance with the superheterodyne method, a low frequency signal including an actual information such as a voice or an image is converted to an intermediate frequency signal and the intermediate frequency signal is transmitted on a carrier signal of a high frequency. The transmitter employing the superheterodyne method has a complex configuration and a large power consumption.
In order to overcome the disadvantages of the superheterodyne method, a direct conversion scheme wherein a base-band signal is up-converted to the carrier signal instead of using the intermediate frequency signal. The direct conversion scheme is advantageous in that the direct conversion scheme has a minimum power consumption of a transmitter structure of the wireless communication system, and the transmitter of the direct conversion scheme may be miniaturized and may be manufactured at a low cost.
However, in accordance with the conventional direct conversion scheme, since a quadrature signal, i.e. I and Q signals should be generated at a local oscillator, more hardware are required to carry out an accurate 90° phase shift of a high frequency signal, resulting in the large power consumption and a degradation of a modulation quality. The modulation quality refers to characteristics such as an EVM (Error Vector Magnitude), a linearity and a power consumption.
That is, in accordance with the conventional direct conversion scheme, since the quadrature signal should be generated based on a local oscillation signal, the generation of the local oscillation signal requires the accurate 90° phase shift of the high frequency signal. Therefore, the conventional direct conversion scheme is a main reason for more hardware requirement, the large power consumption and the degradation of the modulation quality.
Accordingly, a digital intermediate frequency transmitter aimed at a SDR (Software Defined Radio) which allows multiple bands is under a development as a transmitter to be used in the wireless communication system.
FIGS. 1 and 2 are block diagrams illustrating a conventional digital intermediate frequency transmitter.
FIG. 1 is the block diagram illustrating a conventional digital intermediate frequency transmitter using an I/Q path.
As shown, in accordance with the conventional digital intermediate frequency transmitter using the I/Q path, base-band signals BB_I and BB_Q is mixed with a signal generated by an intermediate frequency oscillator 110 by mixers 120a and 120b to be up-converted to I/Q intermediate frequency signals in a digital domain. In this case, a signal that is shifted by 90° by a phase shifter 130a is mixed with the base-band signal BB_I.
The up-converted signals are converted to analog signals using DACs 140a and 140b. 
In this case, a sampling frequency is FS, which is n times the intermediate frequency (where n is an integer no less than 2).
Unwanted harmonic signals generated during the conversion in the DACs 140a and 140b are removed by low pass filters 150a and 150b. 
Thereafter, an I/Q frequency up-conversion is carried out to obtain an RF transmission signal. That is, an output signal of an RF oscillator 110b is mixed with output signals of the low pass filters 150a and 150b by mixers 120c and 120d. 
In addition, a phase of the output signal of the RF oscillator 110b is shifted by a phase shifter 130b to be provided to the mixer 120c. 
The phase shifter 130b or a voltage controlled oscillator having a frequency 2× LO frequency to be divided may be used to generate an I/Q LO.
An undesired side-band signal may be removed by adding or subtracting the up-converted outputs of the mixers 120c and 120d through an operator 160.
After the undesired side-band signal is removed, the signal is amplified through a power amplifier 170, and is transmitted to an antennal by passing through a band pass filter 180.
FIG. 2 is a diagram illustrating a signal in a frequency domain after passing through the operator 160 in the conventional digital intermediate frequency transmitter of FIG. 1.
As shown, the side-band signal is removed by carrying out a subtraction through the operator 160.
FIG. 3 is a block diagram illustrating a conventional digital intermediate frequency transmitter using a single path.
While the conventional digital intermediate frequency transmitter of FIG. 1 uses the I/Q path, the conventional digital intermediate frequency transmitter of FIG. 3 uses the single path. In addition, while the conventional digital intermediate frequency transmitter of FIG. 1 uses two phase shifters 130a and 130b, the conventional digital intermediate frequency transmitter of FIG. 3 uses only phase shifter 130 for a frequency up-conversion.
That is, after the up-conversion to the I/Q intermediate frequency using the phase shifter 130, the operation is carried out by the operator 160 and the digital signal is converted to the analog signal by a DAC 140. Thereafter, an unwanted harmonic signal generated during the conversion in the DAC 140 is removed by a low pass filter 150.
Thereafter, the frequency up-conversion is carried out by a mixer 120e in order to obtain the RF transmission signal. In addition, a filtering is carried out by a band pass filter 190 which is an external element to remove the undesired side-band signal.
Other components are similar to those of FIG. 1.
FIGS. 4a and 4b are diagrams illustrating a signal in a frequency domain in the conventional digital intermediate frequency transmitter of FIG. 3, wherein FIG. 4a illustrate the signal after passing through the operator 160 and FIG. 4b illustrates the signal after passing through the band pass filter 190.
The conventional digital intermediate frequency transmitter described with reference to FIGS. 1 through 4b has following disadvantages.
The conventional digital intermediate frequency transmitter of FIG. 1 has a problem of a mismatching of the I/Q path. Moreover, since the two DACs 140a and 140b and the two low pass filters 150a and 150b are used, the power consumption and an integration area is increased and a manufacturing cost is also increased. In addition, the conventional digital intermediate frequency transmitter of FIG. 1 is disadvantageous in generating the high frequency I/Q LO signals. The conventional digital intermediate frequency transmitter of FIG. 1 also generates an I/Q LO mismatch during the phase shift in the phase shifter 130b, and requires a large hardware resource and power consumption in order to generate accurate I/Q LO signals.
Moreover, since the conventional digital intermediate frequency transmitter of FIG. 3 uses the band pass filter 190 which is the external element, an integration is not possible, and the manufacturing cost is increased due to the high performance high frequency band pass filter 190.